I'm going to concur with selmnoo, but be less rude about it: sorry, but those are the rules of the game. However hard a time you're having finding senior developers, the incentive to produce them domestically gets skewed downwards, making the problem worse in the long term, if we let you "patch" the issue temporarily with immigration rather than signaling higher demand to the market with rising salaries.
Of course, your firm can also do its own part to fix the problem directly: hire more junior candidates and train them up. Yes, I realize this is quite difficult for your situation, but consider that if few to no companies train junior developers into senior ones, that explains why there are so few senior developers on the market.
And then there's the housing shortages around NYC and SF that make land rent eat up most salary rises -- that's worth an entire post in itself.
Training is part of the solution, but I truthfully don't think that can be driven by startups. We might not even be around years from now to receive the rewards from our investment in junior developers.
More practically, I wouldn't feel right hiring junior developers when I don't think we'd be giving them the best training given our current staffing levels and product needs. Since we already have too few senior devs, I'm not sure I could justifiably ask the team to take on the additional burden of mentoring junior developers (though I do hope we can start a robust training program once the dev team is a bit larger).
We of course help each other, but bringing on a junior developer is absolutely considered an investment. There is a tangible decrease in the amount of time senior developers have available for other tasks, not to mention the cost of any off-site training we might invest in.
Using that logic, you could say immigration laws are actually what are making the problem worse in the long term - by temporarily keeping salaries higher than what they otherwise would be.
Of course, your firm can also do its own part to fix the problem directly: hire more junior candidates and train them up. Yes, I realize this is quite difficult for your situation, but consider that if few to no companies train junior developers into senior ones, that explains why there are so few senior developers on the market.
And then there's the housing shortages around NYC and SF that make land rent eat up most salary rises -- that's worth an entire post in itself.